Catalytic wall-flow filter

ABSTRACT

A wall-flow filter ( 10 ) for an exhaust system of a combustion engine comprises: a plurality of channels ( 4,6,8 ) in honeycomb arrangement, wherein at least some of the channels ( 6 ) are plugged ( 12 ) at an upstream end and at least some of the channels ( 4,8 ) not plugged at the upstream end are plugged ( 14 ) at a downstream end; an oxidation catalyst ( 16 ) on a substantially gas impermeable zone at an upstream end of the channels ( 4,8 ) plugged at the downstream end; and a gas permeable filter zone ( 22 ) downstream of the oxidation catalyst for trapping soot, characterised in that in an exhaust system, preferably a diesel exhaust system, the oxidation catalyst is capable of generating sufficient NO 2  from NO to combust the trapped soot continuously at a temperature less than 400° C.

[0001] This application is a divisional of U.S. patent application Ser.No. 09/807,571, filed Jul. 23, 2001 (the entire disclosure of which isexpressly incorporated by reference herein), which is the U.S. NationalPhase of International Application No. PCT/GB00/03064, filed Aug. 14,2000, which claims priority of British Patent Application No. 9919013.4,filed Aug. 13, 1999.

[0002] This invention relates to a wall-flow filter, and in particularto a wall-flow filter including a catalyst.

[0003] Exhaust gases of a combustion engine contain a number ofcomponents linked with damaging health and the environment. One of theseis the soot component. A way of controlling the amount of soot in theexhaust gas is to use a filter downstream from the exhaust manifold, theidea being to burn (oxidise) soot trapped on the filter, therebyregenerating the filter.

[0004] A known type of soot filter is the wall-flow filter. This filtercan be made, for example, from a ceramic monolith including channels ina honeycomb arrangement. A typical embodiment has each channel pluggedat one or other end thereof, and at the opposite end to the laterallyand vertically adjacent channels. When viewed from either end, thealternately plugged and open ends of the channels take on the appearanceof a chessboard. The ceramic material from which the filter can be madehas a pore size sufficient to allow gas permeability so that thepressure drop across the filter is relatively low, but which preventsthe passage of soot. Thus soot is filtered from the exhaust gases.

[0005] EP-A-0341832 and corresponding case U.S. Pat. No. 4,902,487describes a process and treatment system for soot-containing exhaustgas, the gas also containing nitric oxide (NO), which, processcomprising passing the gas unfiltered over an oxidation catalyst toconvert NO to nitrogen dioxide (NO₂), collecting the soot on adownstream filter and combusting the collected soot continuously atunder 400° C. by reaction with the NO₂; and there have been recentproposals to add further steps to that process and system, for examplenitrogen oxides (NOx) removal steps (see EP-A-0758713). EP-A-0341832 andU.S. Pat. No. 4,902,487 describe Johnson Matthey's ContinuouslyRegenerating Trap (CRT™) technology and are incorporated herein byreference.

[0006] In the process described in EP-A-0341832 the oxidation step andthe filter combustion step are carried out in two different honeycombseach in a separate shell or can or mounted within a single can. However,there are problems in adopting either embodiment. A problem with theformer embodiment is that there can be limited space under-floor on avehicle to mount each can. In the latter embodiment, a problem is thatthe construction of the can is complicated. If further downstreamprocess steps are required these problems are exacerbated.

[0007] We have now found that these and other problems can be overcomeor reduced by carrying out each of the treatment steps on a singlewall-flow filter or single “brick”. U.S. Pat. No. 5,089,237 discloses asoot burn-off filter for an exhaust system of a combustion engine, whichfilter includes a porous ceramic honeycomb block having channels pluggedalternately at the ends to define a flow path through the partitionwalls of the channels, the walls at the inlet end having a catalyticcoating. We understand from this document that the filter is used in adiscontinuous process in which soot is allowed to accumulate on thefilter and is periodically burnt off by raising the temperature andensuring that sufficient oxygen (O₂) is available. Owing to thecombustion of a substantial quantity of soot in a relatively smallspace, temperatures high enough to produce destructive effects on thefilter are readily attained. To limit such effect the filter of U.S.Pat. No. 5,089,237 provides a gas-tight region in the partition walls atthe downstream end of the upstream channels. Although the catalyticcoating is stated to lower the temperature at which soot combustiontakes place, it evidently does not make the gas-tight regionunnecessary.

[0008] According to one aspect, the invention provides a wall-flowfilter for an exhaust system of a combustion engine, which filtercomprises: a plurality of channels in honeycomb arrangement, wherein atleast some of the channels are plugged at an upstream end and at leastsome of the channels not plugged at the upstream end are plugged at adownstream end; an oxidation catalyst on a substantially gas impermeablezone at an upstream end of the channels plugged at the downstream end;and a gas permeable filter zone downstream of the oxidation catalyst fortrapping soot, characterised in that in an exhaust system the oxidationcatalyst is capable of generating sufficient NO₂ from NO to combust thetrapped soot continuously at a temperature less than 400° C.

[0009] The term “continuously” means that collected soot is combusted ina continuous exhaust gas flow; it thus excludes (except in the event ofmalfunction) the combustion of a large amount of accumulated soot. Itdoes, however, encompass relatively minor variations in the level ofcollected soot and in the gas composition in response to normalvariations of engine operating conditions and to short-term injection ofreductant or NOx specific reactants in order to remove NOx downstream ofthe filter.

[0010] For the avoidance of doubt, by “absorb” herein, we mean to hold arelevant species on a relevant surface of a body (otherwise “adsorb”)and/or to hold a species below the surface of a body i.e. within thebody.

[0011] Exhaust gases from a combustion engine also contain unburnthydrocarbon (HC), carbon dioxide (CO₂), carbon monoxide (CO), steam(H₂O(g)) and nitrogen (N₂). Its content of NOx comprises NO and NO₂, themajority being NO. At least sufficient NOx should be present to provide,after the catalytic oxidation of NO to NO₂, at least enough NO₂ tooxidise the collected soot as it is formed, or after a smallaccumulation of soot. As described in the above mentioned EP-A-0341832,and in PCT application no. GB00/02062, additional NOx may be supplied bye.g. injecting nitric acid or the product of local oxidation of ammonia(NH₃) or an ammonia precursor, such as urea.

[0012] The filter of the present invention provides a gaseous flow pathdefined in part by the open ends of each channel plugged at thedownstream end, and the pores of the gas permeable wall of the channel.Soot in the gaseous exhaust enters the channel, but cannot pass throughthe pores of the channel wall. Thus soot is filtered from the gaseousexhaust by the pores of the filter. Furthermore, the gaseous exhaust isforced to flow over the entire zone supporting the oxidation catalyst,as the wall of the filter on which it is supported is substantially gasimpermeable. This achieves the best possible yield of NO₂ from NO at agiven temperature and also avoids caking the catalyst in soot.

[0013] Preferably, the arrangement of channels plugged at the upstreamand downstream ends of the filter is such that each channel of thefilter is plugged at one or other end thereof, and at the opposite endto the laterally and vertically adjacent channels, although otherconfigurations can be used. For example, in an arrangement of striatedappearance, a first longitudinal array of channels is plugged at oneend, and the longitudinal arrays of channels either side of the firstarray are plugged at the opposite ends of the filter, and so on. Theprovision of a filter including at least some unplugged channels,thereby to act as a by-pass to the filter channels, is also within theambit of the present invention. The unplugged channels can also be usedto introduce a reactant required downstream of the wall-flow filterzone.

[0014] The channels are preferably square in cross-section but can beany other shape such as circular, rectangular, hexagonal or triangular.

[0015] Preferably, the oxidation catalyst, which is advantageously acompound including a platinum group metal (PGM), such as platinum (Pt)or palladium (Pd), blocks the filter pores in the gas impermeable zone,thereby making the zone substantially gas impermeable. Filter-gradematerials for making the filter suitably have a mean pore diameter inthe range 0.4 to 20×10⁻³ inch (1 to 50 μm) for gas treatments at aboutatmospheric pressure, but other values may be more appropriate forliquids or at higher or lower pressures. Although it is possible tomanufacture a filter in which the filter material itself is gasimpermeable or the pore size is graded so that the upstream part ofchannels plugged at the downstream end are gas impermeable but gaspermeability increases towards the filter zone, we prefer that thefilter material is of uniform pore size and that zones supporting thecatalyst are rendered gas-impermeable by applying materials to them.When the filter is ceramic, it may be the product of shaping (e.g. byextrusion) and a composition containing sufficient fugitive material toleave, once removed e.g. by calcination, the required pores. The filter,whether ceramic or metal, may be the product of moulding and sintering apowder, possibly via foam. Whilst the filter material can be metal, weprefer it to be made of a ceramic material such as cordierite, alumina,mullite, silicon carbide, zirconia or sodium/zirconia/phosphate. Thehoneycomb has typically at least 50 cells per square inch ((cpsi)(7.75cm⁻²), possibly more, e.g. up to 800 cpsi (124 cm⁻²). Generally therange 100-600 cpsi (15.5-93 cm⁻²) is preferred, of which 200-400 cpsi(31-62 cm⁻²) is most preferred.

[0016] According to preferred embodiments, the downstream channels ofthe filter include a catalyst for a NOx absorber (or NOx trap) andoptionally a NOx reduction catalyst or Selective Catalytic Reduction(SCR) catalyst downstream of the NOx absorber. The combination of CRT™technology and the SCR of NOx with a nitrogen-containing compound, suchas NH₃ or urea, is described in our WO 99/39809. In a furtherembodiment, the channels upstream of the oxidation catalyst can includea sulphur oxides (SOx) absorber (SOx trap). The upstream SOx absorbercan be included in embodiments with or without the NOx absorber or NOxreduction catalyst or SCR catalyst. As with the zone including theoxidation catalyst, in embodiments including one or more of the SOxabsorber, NOx absorber or NOx reduction catalyst or SCR catalyst, thezones of each catalyst or absorber is substantially gas impermeable, andthe gas impermeability is preferably provided by the catalyst orabsorber or the catalyst or absorber support.

[0017] The NOx absorber/trap comprises a compound including an alkalimetal, an alkaline earth, rare earth metal or transition metal or amixed oxide capable of forming nitrates and/or nitrites of adequatestability in non-reducing conditions and of evolving nitrogen oxidesand/or nitrogen in reducing conditions, an oxidation catalyst,preferably Pt, and a reduction catalyst, preferably rhodium (Rh).Composite oxides of e.g. alkaline earth metal and copper can also beused, such as (adopting periodic table letter codes) Ba—Cu—O orMnO₂—BaCuO₂, possibly with added CeO₂, or Y—Ba—Cu—O and Y—Sr—Co—O. TheNOx reduction catalyst will generally include one or more PGM, butespecially Pt, Pt/Rh, Pd/Rh or Pt/Pd/Rh. The SCR catalyst can be acopper-based material, Pt, a mixed oxide of vanadia (V₂O₅) and titania(TiO₂) or a zeolite or mixtures of two or more thereof and is preferablyV₂O₅/WO₃/TiO₂. Reference can be made to our WO 99/55446 and WO 99/39809for further information. The SOx absorber can include an alkaline earthmetal oxide or alkali metal oxide or mixtures of any two or morethereof. Reference can be made to our EP-A-0814242 for further details.

[0018] In an exhaust system including an embodiment of the presentinvention including a NOx reduction catalyst or an SCR catalyst, thesystem can include means to inject reductant and/or NOx-specificreactant continuously or intermittently upstream of these catalysts. Theoxidation catalyst of the invention can be in two parts, optimisedrespectively for oxidation of HC and CO and for conversion of NO to NO₂.In the embodiment including both the NOx absorber and reductioncatalyst, they can be in distinct regions of the filter or associated,for example co-precipitated or co-impregnated or present as sandwichedlayers or as relatively fine (e.g. 1-500 μm) particles or overlayed indifferent washcoats.

[0019] Preferably, the or each catalyst or absorber is supported on ahigh-surface oxide support, preferably alumina (Al₂O₃), TiO₂ or zirconia(ZrO₂), but the or each catalyst or absorber can be supported directlyby the filter i.e. without additional high-surface oxide support.

[0020] Desirably, the catalyst(s) and, where present, absorber(s),whether supported on a high surface area oxide or directly by thefilter, increases the local pressure-drop of the wall-flow filter by afactor of at least 2, and preferably up to 10. In one embodiment, thefilter zone itself can be catalysed. A suitable catalyst is an oxidationcatalyst to assist in combusting the soot, but more preferably thecatalyst includes a base metal such as magnesium oxide (MgO) and mostpreferably the catalyst is a combination of lanthanum (La), caesium (Cs)and vanadium pentoxide (V₂O₅). Where the base metal is MgO, a preferredcatalyst is Pt on MgO. If the surfaces of the pores of the filter zoneare to carry a catalyst, the pore diameter may be in the upper fifth ofthe above mentioned range or even higher, but such coating should bethin enough to avoid the need for large pores that would weaken thefilter structurally.

[0021] “Alkali metal” as defined herein includes potassium (K), sodium(Na), lithium (Li), rubidium (Rb) or Cs; “alkaline earth metal” includesbarium (Ba), calcium (Ca), strontium (Sr) or magnesium (Mg); and “rareearth metal” includes cerium (Ce), La or yttrium (Y) or otherlanthamides.

[0022] The filter according to the invention can be packaged in anyconvenient way. Packaging materials commonly used in the art include aceramic or steel wire mesh for wrapping and insulating the filter core;end plugs to prevent exhaust gas leaks through the wire mesh or toprotect the mat; and steel for the shell or can. A suitable header canbe used to provide the transition between the inlet and outlet pipes andthe filter cross-section. Ports for the injection of additives or theintroduction of sensor devices for on-board diagnostics can also beprovided, where necessary.

[0023] The filter of the present invention can be made by methods knownin the art, as exemplified by way of illustration only herebelow. Inthis aspect, the invention provides a method of making a filteraccording to the invention by stage-wise dipping in solutions and/ordispersions of precursors of the catalyst or absorber. In preferredfeatures of this method, the method includes the steps of coating a wallof a channel of a wall-flow monolith with at least one materialeffective to decrease locally the gas permeability of the wall; and thenapplying to the coated wall at least one catalyst or absorber andoptionally calcining the coated monolith.

[0024] However, we prefer to use the apparatus and method described inour WO 99/47260. To this end according to a further aspect, theinvention provides a method of making a filter according to theinvention comprising, in either order, the steps of: (i) locating acontainment means on top of a wall-flow monolith having a plurality ofchannels in honeycomb arrangement, wherein at least some of the channelsare plugged at an upstream end and at least some of the channels notplugged at the upstream end are plugged at a downstream end; and (ii)dosing a pre-determined quantity of a liquid being a washcoat slurry ora solution of a catalyst or catalyst precursor, or a mixture of the two,into the containment means; and then (iii) by applying pressure or avacuum, drawing said liquid component into at least a portion of theopen wall-flow monolith channels, and retaining substantially all of thequantity within the channels.

[0025] In an alternative embodiment, the above method is applied to anunplugged monolith and the channels are plugged after the coatings havebeen applied. If one end of a channel is to be plugged, the methodincludes the step of removing coating applied to that end prior toplugging. Alternatively, the method includes the step of coating onlythe ends of channels which are predetermined to remain unplugged, i.e.the coating is not applied to an end of a channel which is to beplugged.

[0026] In a preferred feature, the method according to the inventioncomprises the step of applying a resist to a region where application ofa washcoat or a solution or suspension of a catalyst or catalystprecursor or mixture thereof is to be delayed. The resist can be a waxor stearic acid, for example.

[0027] According to a further aspect of the invention, there is provideda combustion engine including an exhaust system having a filteraccording to the invention. Preferably, the combustion engine is adiesel engine.

[0028] In another aspect, the invention provides a vehicle fitted with acombustion engine, preferably a diesel engine, including an exhaustsystem having a filter according to the invention.

[0029] According to a further aspect of the invention there is providedthe use of a filter according to the invention for treating exhaustgases from a combustion engine, preferably a diesel engine.

[0030] In a further aspect, the invention provides a process forremoving by combustion soot deposited on a filter disposed in an exhaustsystem of a combustion engine wherein exhaust gas containing NO isinitially passed without filtering over a n oxidation catalyst toconvert NO in the exhaust gas to NO₂ prior to filtering to remove sootand wherein the exhaust gas containing NO₂ is then used to combust thesoot trapped on the filter, the amount of NO converted to NO₂ beingsufficient to enable combustion of soot trapped on the filter to proceedat a temperature less than 400° C., characterised in that the filter isa wall-flow filter, which filter comprises: a plurality of channels inhoneycomb arrangement, wherein at least some of the channels are pluggedat an upstream end and at least some of the channels not plugged at theupstream end are plugged at a downstream end; the oxidation catalyst ison a substantially gas impermeable zone at an upstream end of thechannels plugged at the downstream end; and a gas permeable filter zonedownstream of the oxidation catalyst for trapping soot.

[0031] In order that the invention may be more fully understood,reference will be made to the accompanying drawing which shows anelevated cross-sectional view of one embodiment of a filter according tothe present invention.

[0032]FIG. 1 shows three adjacent channels 4, 6, 8 of a cordieritehoneycomb filter 10, having 200 cpsi (31 cm⁻²) and, before use, pores ofapproximately 10 μm in diameter. Channel 6 is plugged at 12 at theupstream end of the filter 10 and the other two channels 4,8 are eachplugged at 14 at the downstream end of the filter 10. Thus, gas enteringchannel 6 at the upstream end must pass through the walls of the channel6 to reach the downstream end of the filter 10. Over a region or zoneextending downstream of the opening to channel 6, the walls carrycoating 16 comprising an Al₂O₃ washcoat support and a metallic Ptoxidation catalyst. Coating 16 obstructs the pores of the filter walls.Over a region or zone extending upstream from the outlet to filter 10,channels 4 and 8 carry coatings 18, 20 comprising a NOx absorbercomposition including barium oxide (BaO) and a NOx reduction catalystcomposition comprising Pt/Rh. In order to make the coatings 18, 20, thezone to be coated with coating 20 is first covered with a resist, suchas wax or stearic acid, during application of coating 18. In thisembodiment the regions of the cordierite filter 10 including coatings 16and 18 are rendered gas impermeable by the presence of the coating.Between coatings 16 and 18, the walls of the channels 4, 6, 8 remain gaspermeable and provide a filter zone 22.

[0033] Diesel exhaust gas entering the reactor undergoes oxidation ofHC, CO and NO in presence of coating 16. Soot in the gas is collected onthe walls of channels 4, 6, 8 at filter zone 22 and is combusted by NO₂derived from the oxidation of NO. The gas, containing soot combustionproducts, passes through the wall of the filter and contacts the NOxtrap coating 18, which absorbs NO₂. When sufficient NO₂ is stored (asthe nitrate, for example), which can be ascertained by the on-boarddiagnostics of the vehicle, coating 18 can be regenerated with a richpulse of gaseous exhaust i.e. gas including excess reductant such as HC.The rich/lean cycling can be controlled using the vehicle's enginemanagement system. The resulting NOx-rich gas contacts coating 20including the NOx reduction catalyst, which effects oxidation of HC andCO and reduction of NOx to N₂. Alternatively, where coating 20 is a SCRcatalyst, NOx-specific reactant such as ammonia can be injected at anupstream end of the filter at a rate and temperature permittingunreacted ammonia to slip oxidation catalyst 16 and contact absorber 18,which it regenerates, and catalyst 20, over which it reduces NOx to N₂.

[0034] In order that one method of manufacture of the filter accordingto the invention may be more fully understood, the following Example isprovided by way of illustration only.

[0035] The substrate is a filter grade cordierite honeycomb monolith ofsquare cross-section channels 30 mm in diameter and 150 mm long andhaving a mean pore diameter 10 μm in which half of the passages bothends of the monolith are plugged so that each channel of the monolith isplugged at one or other end thereof, and at the opposite end to thelaterally and vertically adjacent channels.

[0036] One end of the monolith is labelled ‘inlet’ and is dipped 25 mmdeep into an aqueous dispersion of hydrated Al₂O₃, then withdrawn, driedat 100° C. and allowed to cool. The inlet end is then dipped to the samedepth in an aqueous solution of 2% w/w platinum chloride. The monolithis dried as before.

[0037] The unlabelled end of the monolith is prepared by firstly dippingit to a depth of 25 mm in turn in an aqueous solution of sodium stearateand then drying the resulting monolith at 100° C. The resulting monolithis then dipped in aqueous hydrochloric acid and then water (twochanges), to rinse off solubles. This procedure produces an insolublestearic acid layer as a resist, which excludes any materials applied inneutral or acidic solution. The resist-coated monolith is then dipped toa depth of 50 mm, into an aqueous solution of barium acetate andplatinum chloride, and then dried. This dip applies these materials toan area upstream of the resist-coated area. Then the coated monolith isdipped in 5% w/w aqueous sodium hydroxide to a depth of 25 mm todissolve the stearic acid resist. This step is repeated twice and isfollowed by two rinses with water. The coated monolith is then dried.The resulting monolith is then dipped to a depth of 25 mm in the Al₂O₃dispersion used at the inlet end, and described above. The coatedmonolith is then dried. Finally, to a depth of 25 mm in Pt/Rh solution.The coated monolith is then dried.

[0038] The resulting monolith is then calcined at 500° C. for 1 hr toconvert the metal salts to oxides or metals and to develop the surfacearea of the Al₂O₃ to provide a filter according to the invention.

What is claimed:
 1. A method for treating soot-containing exhaust gaswith a wall-flow filter comprising the steps of: (i) passing an exhaustgas over an oxidation catalyst disposed on a gas-impermeable zone alongthe wall-flow filter to convert an amount of NO in the exhaust gas toNO₂, wherein the wall-flow filter comprises a honeycomb arrangementdefining: (a) a plurality of first channels plugged at an upstream end,and (b) a plurality of second channels not plugged at an upstream end,but plugged at a downstream end; (ii) thereafter, trapping soot from theexhaust gas in a catalyst-free gas-impermeable zone disposed along theplurality of second channels; and (iii) continuously combusting thetrapped soot generated from step (ii) with the NO₂ produced from step(i), wherein the amount of NO₂ generated from step (i) is sufficient toenable combustion of the trapped soot in the catalyst-freegas-impermeable zone disposed along the plurality of second channelsplugged at the downstream end at a temperature less than 400° C.
 2. Themethod according to claim 1, wherein the wall-flow filter is made of aceramic material selected from the group consisting of cordierite,alumina, mullite, silicon carbide, zirconia andsodium/zirconia/phosphate.
 3. The method according to claim 1, whereinthe plurality of first channels and the plurality of second channels aresquare, circular, rectangular, hexagonal or triangular in cross section.4. The method according to claim 1, wherein the arrangement of theplurality of first channels and the plurality of second channels is suchthat laterally and vertically adjacent channels are plugged at oppositeends.
 5. The method according to claim 1, wherein the honeycombarrangement further comprises a plurality of third channels which areunplugged to provide a flow-through by-pass to the first and secondchannels.
 6. The method according to claim 1, wherein the oxidationcatalyst includes a platinum group metal.
 7. The method according toclaim 6, wherein the platinum group metal is selected from the groupconsisting of Pt, Pt/Rh, Pd/Rh or Pt/Pd/Rh.
 8. The method according toclaim 6, wherein the platinum group metal comprises at least one of Ptand Pd.
 9. The method according to claim 1, further comprising after theexhaust gas passes through the gas permeable zone of the plurality ofsecond channels, the step of absorbing NOx with a NOx absorber disposedalong the gas impermeable zone of the plurality of first channelsplugged at the upstream end.
 10. The method according to claim 9,wherein the NOx absorber is selected from the group consisting of analkali metal, alkaline earth metal, rare earth metal, and mixtures ofany two or more thereof.
 11. The method according to claim 9, whereinthe NOx absorber includes an oxide of at least one of an alkali earthmetal and an alkaline earth metal.
 12. The method according to claim 11,wherein the alkali metal is potassium, sodium, lithium, rubidium orcaesium or a mixture of any two or more thereof, the alkaline earthmetal is barium, calcium, strontium or magnesium or a mixture of any twoor more thereof.
 13. The method according to claim 8, further comprisingafter the step of absorbing the NOx, the step of reducing NOx with areduction catalyst or a Selective Catalytic Reduction (SCR) catalystdisposed downstream of the NOx absorber along the gas impermeable zoneof the plurality of first channels plugged at the upstream end.
 14. Themethod according to claim 13, wherein the step of reducing NOx isthrough use of the reduction catalyst
 15. The method according to claim14, wherein the NOx reduction catalyst comprises at least one platinumgroup metal.
 16. The method according to claim 13, wherein the step ofreducing NOx is through use of the SCR catalyst
 17. The method accordingto claim 16, wherein the SCR catalyst includes copper-based materials,Pt, a mixed oxide of vanadium and titanium or a zeolite or mixtures oftwo or more thereof.
 18. The method according to claim 16, wherein theSCR catalyst comprises V₂O₅/WO2/TiO₂.
 19. The method according to claim1, further comprising before the step of passing the exhaust gas overthe oxidation catalyst, the step of absorbing SOx on the gas-impermeablezone along the plurality of second channels.
 20. The method according toclaim 19, wherein the SOx absorber includes an alkaline earth metaloxide or alkali metal oxide or mixtures of any two or more thereof. 21.The method according to claim 20, wherein the alkali metal is potassium,sodium, lithium, rubidium or caesium or a mixture of any two or morethereof, and the alkaline earth metal is barium, calcium, strontium ormagnesium or a mixture of any two or more thereof.
 22. The methodaccording to claim 1, wherein the wall-flow filter further comprises ahigh-surface oxide support comprising alumina, titania or zirconia forsupporting the oxidation catalyst.
 23. The method according to claim 19,wherein the support supporting the catalyst renders the zone includingthe oxidation catalyst substantially gas impermeable.
 24. The methodaccording to claim 1, further comprising increasing the local pressuredrop of the wall-flow filter by a factor of at least 2 with the presenceof the oxidation catalyst.
 25. The method according to claim 1, furthercomprising increasing the local pressure drop of the wall-flow filter bya factor of at least 10 with the presence of the oxidation catalyst. 26.A method according to claim 1 for treatment of soot-containing exhaustfrom a combustion engine.
 27. A method according to claim 1 fortreatment of soot-containing exhaust from a diesel engine.
 28. A processfor treating soot-containing exhaust gas comprising the steps of firstpassing the soot-containing exhaust gas over an oxidation catalyst toconvert NO in the exhaust gas to NO₂, then filtering the gas to depositsoot on a filter disposed in an exhaust system of a combustion enginewherein the exhaust gas containing NO₂ is used to combust the soottrapped on the filter, the amount of NO converted to NO₂ beingsufficient to enable combustion of soot trapped on the filter to proceedat a temperature less than 400° C., wherein the filter is a wall-flowfilter comprising: a honeycomb arrangement defining a plurality of firstchannels plugged at an upstream end and a plurality of second channelsnot plugged at the upstream end but plugged at a downstream end; theoxidation catalyst on a substantially gas impermeable zone at anupstream end of the second channels; and a catalyst-free gas permeablefilter zone downstream of the oxidation catalyst for trapping soot. 29.The method according to claim 28, for treating exhaust gases from acombustion engine.
 30. The method according to claim 28, for treatingexhaust gases from a diesel engine.